First, I need to correct a previous post - the adjustments are measured in cents, not semitones. Sorry about that, my bad! I found the best way to practice was to do it with another trumpet player. One plays the bottom note the other plays an interval above, both playing at a mp volume. Using the amount of cent adjustment as indicated in the book, the upper note player adjusts their pitch until the resulting tone is in tune with the notes that are being played. Then switch - the player playing the bottom note adjust to the upper note, listening for the resulting tone. Think "Chase the beats away." Resulting tones are always there - they're a mathematical certain. Practicing in this way at a soft volume in a quiet setting is ideal, although there are some accoustical settings that make it very difficult to hear the resulting tone. That doesn't matter - they're still there. Another benefit of practicing to play in tune, especially as described above, is you have to have solid control of your sound (air & embouchure). If you're not playing in a manner where you can make adjustments while playing, or have the willingness to make adjustments, it's next to impossible to play in tune. Sometimes it's as simple as using your 1st or 3rd slide or an alternate fingering. Sometimes it's not. Practicing the Stamp bending exercises is a great way to develop air & embouchure control. The worst recording session I ever did - the sax player put his tuner on the stand and yes, while he played every note "in tune" with the tuner, whenever he had the 3rd or 7th of the chord - fuggedaboudit. Drove the trombonist & me crazy. No matter what we said, he kept claiming "But I'm playing in tune!!" The producer didn't have an ear and didn't care. He just wanted to get the project on tape. I felt sorry for the poor engineer in the booth. A definite "shut up, take the money & run" situation.